build: Fix broken libheif < 1.13 (AcademySoftwareFoundation#3970)

Typo introduced in AcademySoftwareFoundation#3894 broke our ability to build against libheif <
1.13. A conditional compilation that should have kicked in for >=1.16
was mistakenly written as 1.6.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

src/heif.imageio/heifinput.cpp

@@ -62,7 +62,7 @@ class HeifInput final : public ImageInput {
 void
 oiio_heif_init()
 {
-#if LIBHEIF_HAVE_VERSION(1, 6, 0)
+#if LIBHEIF_HAVE_VERSION(1, 16, 0)
     static std::once_flag flag;
     std::call_once(flag, []() { heif_init(nullptr); });
 #endif